Field of the Invention
This invention relates to intercalation pastes that contain precious metal particles, intercalating particles and organic vehicle.
Intercalation pastes may be used to improve the power conversion efficiency of solar cells. Silver based intercalation pastes are printed on aluminum layers that have moderate peel strength after firing and subsequent soldering to a tabbing ribbon. Such pastes may be especially well-suited for use in silicon based solar cells that use aluminum back-surface fields (BSF). Typically, between 85-92% of the rear surface area of the silicon wafer of commercially produced mono- and multi-crystalline silicon solar cells is covered by an aluminum particle layer, which forms a back-surface field and makes ohmic contact to the silicon. The remaining 5-10% of the rear silicon surface is covered by the silver rear tabbing layer, which does not produce a field and does not make ohmic contact to the silicon wafer. The rear tabbing layer is primarily used to solder tabbing ribbons to electrically connect solar cells.
It is estimated that the power conversion efficiency of the solar cells is reduced by 0.1% to 0.2% on an absolute basis when a silver layer makes direct contact to the silicon substrate on the rear side of a solar cell instead of contacting the aluminum particle layer on the substrate. Therefore, it is highly desirable to cover the entire back portion of the solar cell with an aluminum particle layer and still be able to connect solar cells together using tabbing ribbons. In the past, researchers have tried printing silver pastes directly on top of the aluminum particle layer, but during firing in air at high temperatures the aluminum and silver layers interdiffuse, and the resulting layer surface becomes oxidized and loses solderability. Some researchers have attempted to change the atmospheric conditions to reduce oxidation; however, the front side silver pastes perform best in oxidizing atmospheres such as dry air, and overall solar cell efficiency is reduced after processing in inert atmospheres. Other researchers have attempted to lower the peak firing temperature of the wafer to reduce interdiffusion, but front side silver pastes require high peak firing temperatures (i.e., more than 650° C.) to fire through silicon nitride to make ohmic contact to the silicon substrate. Recently, researchers have used ultrasonic soldering of tin alloys directly on top of aluminum to create a solderable surface. This technique has achieved adequate peel strength (i.e., 1-1.5 N/mm) but requires additional equipment and uses a large quantity of tin, which adds cost. Furthermore, using ultrasonic soldering on brittle materials such as aluminum and silicon wafer can increase wafer breakage and decrease processing yields.
There is a need to develop printable pastes that can modify the material properties of underlying metal particle layers during firing. For example, precious metal containing pastes that can be directly printed on aluminum and fired using standard solar cell processing conditions could improve solar cell efficiency. These pastes should reduce Ag/Al interdiffusion in order to remain solderable to the tabbing ribbon. It is desirable for the paste to be screen-printable and act as a drop-in replacement, which would result in no additional capital expenses and can be immediately integrated into existing production lines.